In general, semiconductor elements including transistors, diodes, integrated circuits, and large scale integrated circuits are encapsulated with resinous materials, typically epoxy resins. However, semiconductor elements encapsulated with conventional resinous materials often suffer from deterioration by water and ionic contaminants which can penetrate through the resin encapsulants. One effective solution is to provide semiconductor elements with protective coatings of polyimide resins having excellent heat resistance, electrical properties, and mechanical properties before sealing with resinous encapsulants.
However, although polyimide resins are generally heat resistant, they are insoluble in solvents except some high-boiling organic solvents. It is thus a common practice to form a polyimide resin coating by dissolving a polyamic acid which is a polyimide precursor in an organic solvent to form a coating solution, applying the solution to a semiconductor element, and heating the coating for curing (imidization). This method requires heat treatment at 300.degree. C. or higher temperatures for extended periods of time for converting polyamic acid into polyimide before polyimide resin coatings are formed. The high-temperature, long-term heating step is a disadvantageous working step, especially from the standpoint of energy consumption. If heating is insufficient, then some polyamic acid would be left in the resulting resin structure, resulting in the polyimide resin being less resistant against humidity and corrosion. Particularly for insulating protective coatings on semiconductor elements, such losses of resin attributes would undesirably deteriorate semiconductor elements to reduce their service life.
Several methods are known in the art for the preparation of polyimide resins soluble in organic solvents. One method is by heating tetracarboxylic acid dianhydrides and aromatic diamines in such solvents as phenol and halophenols for reaction to thereby form polyimide resins soluble in phenolic solvents (e.g., Japanese Patent Publication Nos. 26878/1972, and Jap. Pat. Appln. Kokai 65227/1980, 187430/1983, 35026/1985, and 197731/1985). Another method is by reacting specific tetracarboxylic acid dianhydrides and specific diamines to form polyimides soluble in polar solvents having a high boiling point such as N-methyl-2-pyrrolidone (e.g., Japanese Patent Publication No. 30319/1977, and Jap. Appln. Kokai 83228/1986, and 18426/1987).
The polyimide resins prepared by the former method require careful handling and are less safe or hygienic in that when they are used as solutions in phenolic solvents to form coatings, evaporation of the solvent gives off a foul smell like cresol odor and the solvent causes chemical scald to the skin on accidental contact therewith. In turn, the polyimide resins prepared by the latter method have the problem that when they are applied to substrates as solutions in N-methyl-2-pyrrolidone which is highly hygroscopic, the coatings become white turbid due to moisture absorption, resulting in a loss of film strength. In addition, no improvement in working conditions is expected because N-methyl-2-pyrrolidone solvent has a high boiling point so that high-temperature, long-term heating is necessary to completely remove the solvent. Thus, these resins are against the purpose of forming polyimide films of quality by low-temperature, short-term heating.
Furthermore, in applying polyimide resin solutions to semiconductor elements by these techniques, the semiconductor elements at the surface tend to repel the applied solutions, often failing to coat necessary sites or to form an even coating due to varying coverage. Particularly when steps and ramps are present in semiconductor elements, the resin coatings deposited at such sites are too thin to provide semiconductor protection and often contain pinholes and other defects. All these factors lead to a loss of reliability of semiconductor elements.
For reducing variations in coating thickness, Japanese Patent Application Kokai 56452/1984, 56453/1984, and 56454/1984 disclose semiconductor element protecting compositions in which various fillers are blended in polyimides soluble in organic solvents. These compositions also suffer from the solvent related problems because phenol, N-methyl-2-pyrrolidone and similar solvents are used.
Therefore, an object of the present invention is to provide a novel and improved semiconductor element protecting composition comprising a polyimide resin which is dissolvable in a low-boiling, volatile organic solvent to form a solution which is ready to form a polyimide resin film having improved adherence, heat resistance, electrical and mechanical properties through a low-temperature, short-term heating step. Another object of the invention is to provide such a protecting composition which is shelf stable and safe. A further object of the invention is to provide such a protecting composition to which semiconductor elements on the surface show improved receptivity so that uniform defect-free films may form even around steps and ramps on the elements.